THE PEOPLE of Puerto Rico were voting in a referendum yesterday on whether their Caribbean island, which fell into American hands 100 years ago as spoils of the Spanish-American War, should seek to become fully integrated into the United States.

As polling came to a close last night the result was too close to call. Ballot papers offered voters five options, including one for independence as a sovereign nation. Only two of the options were expected to win significant support: one to petition the US for full statehood and another to retain the island's current status as a commonwealth of the US.

The issue is emotive for the island, which has a population of 3.8 million. Supporters of statehood argue that, as a commonwealth, Puerto Ricans are second-class citizens who live under American cultural domination without the chance to vote in US elections or affect American politics.

Leading the statehood camp is the Governor, Pedro Rossello. "Keep the political inferiority, keep the economic limitations, keep the social dependency, that's what commonwealth has meant to Puerto Rico as a transitory and territorial status," he said.

But opponents of the campaign believe that, as a commonwealth, the island has the best of all worlds - close association with the US and the benefits of federal funding while at the same time retaining a patina of national identity. Puerto Rico, for instance, has its own Olympic team.

The island was under Spanish rule for 400 years before it was ceded to the US in 1898. The present commonwealth status was conferred on Puerto Rico in 1952. Yesterday's was the third referendum on statehood in 30 years. The last time, in 1993, statehood lost by two percentage points.

Victory for the statehood campaign would mark only the start of a difficult constitutional battle.

To become the 51st state of America, Puerto Rico would have to win the support of the US Congress, a task that would probably not prove easy. Members of Congress may worry, for example, about the impact on the Union of adding to it a state that is entirely Spanish- speaking.